How can I become representative payee for mom's SS check? She is prone to scammers.

It will be easier over all to become POA and just have the power or all accounts, but it means you are responsible then for every penny in and every penny out, and it also means that this is what your elder wishes done. Then you can give the elder a small checking account of her own which she can manage on her own. This can only be done if the elder wants it, as I said, because if she isn't diagnosed with dementia severe enough to mean she can't take charge of her finances, there is nothing you can do about her poor judgement. We are allowed in the USA to have poor judgement and to waste all out money (unfortunately).
If you have POA and are managing the account the money comes automatically into then you are managing and saving her SS for her. To get POA also doesn't mean the SS will let you be in charge of her funds from SS. It is a process of having to prove the person incompetent, filing to act for her as payee, and there are many things you must do AS payee. Look it up on the Federal Social Security website.

It can be done online if she has a computer account

How do I become a payee in South Dakota?
The person asking to be named as the representative payee must file an application with the Social Security Administration. That person must also supply evidence that he or she is related to or cares for the beneficiary of the payments.

You must complete form SSA-11 (Request to be selected as payee) and show us documents to prove your identity. You will need to provide your social security number, or if you represent an organization, the organization's employer identification number.